Dyeing keratinous fibres, especially human hair, has become a routine method to accommodate the changes in fashion and style and to provide coverage of grey hair.
Numerous products are available to perform dyeing and a great variety of shades and tones can be achieved for almost any colour and type of hair.
Nevertheless, once virgin hair has been coloured, subsequent colouring events are found to be difficult in providing either the same overall colour appearance or natural colour variations.
Particularly, the tips of coloured keratinous fibres exhibit more damage than the root portion. This is not only due to the high number of dyeing events that the tip portions have undergone. The combination of reduced delivery to the tips of moisturizing and emollient substances naturally secreted by sebaceous glands of the scalp, the increased exposure to the aggressive environmental conditions and the rubbing against clothes, especially in long hair, are additional circumstances which are also known to cause further hair tip damage.
Damaged keratinous fibres present a heterogeneously altered structure of the outer hydrophobic layer, creating variable substrates for the next colouring event. This variety of substrates results in altered uptake of dye into the fibres during dyeing. Virgin or less coloured keratinous fibres, as those found in the roots, uptake dyes differently from those in the porous tips that have been undergoing several dyeing events. The outcome is the formation of very different and unexpected colours, resulting in an overall effect which is unlikely to be desirable.
Efforts in applying the dyeing compositions for a longer time period onto the root than onto the tips have been confirmed to be inefficient to provide a homogenous root-to-tip evenness and do not particularly improve targeted grey coverage.
Attempts to reformulate the conventional oxidative dye compositions already available with ingredients capable of providing homogenous root-to-tip evenness, targeted grey coverage or natural colour variation are difficult and expensive.
Often reformulations of the conventional chassis lead to dye/medium phase separation, pH variation, by-products and heat formation, high degradation of dyes and formation of highly irritant intermediates. Besides, reformulations of the conventional chassis are unlikely to deliver the traditional colours and tone shades, which are appreciated and expected by habitual users.
Finally, cosmetic compositions that are used on human hair, besides providing the aesthetic expected results must be unobjectionable in regard to toxicological and dermatological properties and must provide fastness to a permanent wave treatment, acid fastness and fastness to rubbing.
To overcome the above mentioned problems, different approaches have already been proposed in the art. WO 99/55295 discloses a method to treat mammalian wet or dry hair with a composition comprising a hydrophobic and/or cationic conditioning agent. U.S. Pat. No. 6,497,888 discloses a process to limit the penetration into skin and/or keratinous fibres of active agents by applying to the skin and/or to keratinous fibres a composition comprising a dispersion of vesicles enclosing a ceramide. U.S. Pat. No. 5,500,218 discloses a preparation for preventing the colouring of skin adjoining the hair line when dyeing the hair. Said preparation is hydrophobic, viscous and has glue-like consistency as it comprises polyethylene glycols, alkoxylated fatty acids and polyols, which makes it a waxy barrier to the dye. EP 1238649 discloses a hair dye composition to be used at a pH of from 2 to 6 comprising a compound having a 5 to 6 membered lactone skeleton and an acid dye.
However, there still remains a desire to provide methods to pre-treat the hair prior to a dyeing event to obtain root-to-tip evenness, targeted grey coverage and/or natural colour variation. The methods should be flexible enough to control and modulate dye uptake and to be used with the conventional oxidative dye chassis, which work at basic pH without having to reformulate the already commercially available dyeing compositions.